Rapid and recent allopatric speciation in firs from central Mexico:
evidence for reinforcement?
Abstract
Secondary contact of species that have evolved partial reproductive
isolation in allopatry may result in several outcomes, which range from
rampant hybridization to barrier reinforcement. Reinforcement arises
from reduced hybrid fitness, which promotes assortative mating and hence
speciation. In plants, self-fertilization and disjunctions in
reproductive-phenology are often invoked as evidence of reinforcement.
However, local adaptation and pleiotropic effects during colonization
can also lead to reproductive isolation without reinforcement. We
explored these possibilities in a fir species complex (Abies flinckii -
A. religiosa) distributed in ‘sky-islands’ along the Trans-Mexican
Volcanic Belt (TMVB), in central Mexico. Despite co-occurring in two
independent sympatric regions (west and center), these two taxa seem to
rarely interbreed because of disjunct reproductive phenologies. We
genotyped 1,147 SNPs, generated by GBS across 23 populations, and
compared multiple demographic scenarios, built based on the geological
history of the TMVB. The best-fitting model suggested a recent species
split (for a conifer), dating back to ~1.2 Ma, together
with early asymmetric gene flow (mostly from A. flinckii into A.
religiosa), limited to the central sympatric region. Coupled with the
lack of support for colonization models, the summary statistics (f,
Hobs, FST, θπ, etc.) and historical demographic inferences made herein
point to a rapid speciation with an early development of reinforcement,
as a putative mechanism for avoiding hybridization. The role of
reinforcement should be thus further explored in the (sub)tropics, as
likely explanation for how species diversity is generated and
maintained.